The present invention relates to improvements of a cover glass structure of a flat type fluorescent indicator tube.
As well known, the flat type fluorescent indicator tube is generally comprised of a substrate, display sections each consisting of a plurality of anode segments disposed on the substrate for displaying alphanumeric characters, a cathode filament stretched above and across those display sections, a box-like glass envelope or cover glass disposed on the substrate for hermetically enclosing the display sections and the cathode filament disposed, connecting means extending to the outside of the glass envelope with connection to the anode segments and the cathode filament, and a exhaust tube sandwiched between the glass envelope and the substrate and communicating with the space defined by the glass envelope and the substrate. The exhaust tube which is generally tubular is fitted at one end in the side wall of the glass envelope with the other end sealed. The other end of the exhaust tube is opened when the exhaust tube is fitted to the glass envelope and is sealed after the space in the glass envelope has been evacuated through the exhaust tube. The side wall of the glass envelope is provided with a coupling portion so shaped as to permit the exhaust tube to be fixedly received therein.
Generally, one end portion of the exhaust tube is placed in the coupling portion of the glass envelope and then the gap between the exhaust tube and the coupling portion is filled with glass frit to glue the exhaust tube to the glass envelope into a integral form. Then, the glass envelope with the exhaust tube fixed thereto is secured onto the substrate by using a glass frit. Alternately, the exhaust tube is placed on the substrate and then glass envelope is placed on the same with the exhaust tube with an alignment of the coupling portion of the glass envelope with the exhaust tube on the substrate. Following this, those elements are integrally glued together by using frit glass. In assembling, the exhaust tube frequently damages the anode segments disposed on the substrate or the cathode filament stretched above the substrate. Further, the connection of the exhaust tube with the glass envelope is often insufficient so that an air leak tends to occur. Several attempts have been made to overcome those defects. For example, specially devised instrument has been used to align the exhaust tube with the glass envelope; however, the result is poor.
The glass envelope is transparent and flat in order that the alphanumeric characters can be seen from the exterior without any distortion. However, while the coupling portion is formed on the side wall of the glass envelope, the flat glass envelope is distorted and the distorted alphanumeric characters which are displayed by the anode segments are seen through the distorted glass envelope. The distortion of the glass envelope arises from the fact that, in the assembling process of the glass envelope, while the coupling portion is formed by means of softing a flat glass, the compression exerting on the coupling portion of the softened glass envelope is carried to the flat portion of the glass envelope on the display sections, resulting in the distortion thereof.